1. Field of the Invention:
The present invention relates generally to power supply and output circuits for use with a microphone, and is directed more particularly to power supply circuits for use with a microphone unit having an electrostatic transducer and a pre-amplifier transistor.
2. Description of the Prior Art:
In an electrostatic transducer, especially a microphone, it is necessary to supply DC voltage to a pre-amplifier formed of a field effect transistor.
In general, a microphone assembly of small size is made of a microphone unit consisting of a diaphragm and a pre-amplifier as one block, a power supply for supplying power to the microphone unit, and a cable for coupling between the two.
There are two methods for supplying power to such a microphone of small size. One of the methods is with a power supply and output circuit having a battery for supplying DC power to the preamplifier in the microphone unit and also having an output transformer to which is fed the microphone output signal. The power and output signal both travel through a single core cable and, in the power supply, the output signal passes through the battery to a signal output terminal. The other method is by use of a system in which an output transformer is included in the microphone unit and the power supply consists primarily of a battery. In this case, the microphone unit is connected to the power supply through a two-core cable and a signal from the microphone proper is delivered to an output terminal of the power supply without passing through a battery.
As mentioned above, in the prior art a single core cable or two-core cable is used in accordance with either but not both of the two power supply systems.